1. Field of the Invention
The present invention relates to a wiring substrate and, more particularly, to connection between an external substrate and a multilayered wiring substrate on which one or a plurality of-integrated circuits and circuit parts are mounted.
2. Description of the Related Art
Conventionally, an integrated circuit is stored in a container called a package singly or together with several parts to facilitate handling of the integrated circuit and not to degrade the characteristics of the integrated circuit. The package is connected to an external substrate by a pin grid array, a flat lead, a flexible wiring substrate, or the like. For example, packages using the pin grid array or the flat lead are already on the market.
A packaging system for storing a substrate on which a plurality of integrated circuits or circuit parts are mounted in one container is called a multi-chip module (MCM). This MCM is used to increase a packing density of the parts and to realize a compact apparatus having high performance. An important problem of the MCM is that a large number of parts are efficiently arranged in a limited area.
Conventionally, an MCM is connected to an external substrate by a pin grid array, a flat lead, or a flexible wiring substrate as in a package.
For example, in Jpn. Pat. Appln. KOKAI Publication No. 1-272140, an MCM in which an Si wiring substrate is fixed on an A1N package is disclosed. In this case, signal inputs/outputs using a lead frame are shown. In Jpn. Pat. Appln. KOKAI Publication No. 2-184052, an MCM in which a sapphire substrate is mounted on a ceramic substrate is described. In this case, a connection method using contact pins is described.
As the arrangement of an MCM, in addition to the above method using a substrate on which a plurality of elements are mounted and a container, the following method is known.
For example, in a silicon-on-silicon MCM (IBM (corp)), a chip is directly mounted on a silicon substrate (IMC, 1992, proceeding, P533). In this case, input/output operations are performed using a flexible wiring substrate.
According to another MCM, a wiring portion having an organic polymer layer as an insulating layer is formed on a ceramic multilayered wiring substrate comprising an aluminum nitride sintered body, and an integrated circuit, a circuit part, and the like are mounted on the wiring portion or the ceramic substrate. In this MCM, a cap is formed on the ceramic substrate and terminals connected to an external circuit are formed. In this case, the ceramic substrate is used as a part of a package. In this MCM, the size of the ceramic multilayered wiring substrate comprising the aluminum nitride sintered body is the maximum size of the MCM.
In any of the methods described above, an important problem of an MCM is that a large number of parts are effectively arranged in a limited area.
When a signal is to be extracted from a surface on which parts are mounted, the area of portions on which pins or leads are mounted restricts an area for mounting parts. When the number of signals is increased, especially in a method using pins, when the size of a package or an interval between pins is not changed, the area of the pin portions is increased, thereby further decreasing the area of portions for mounting parts.
In an MCM in which a multilayered wiring portion having an organic polymer layer as an insulating layer is arranged on a ceramic wiring substrate comprising aluminum nitride or an aluminum nitride sintered body or a wiring substrate comprising another an insulator, the following problems are posed.
That is, pins or leads are generally connected by brazing. This brazing includes the step of heating the connection portions of the pins and leads to about 800.degree. C. Therefore, the brazing step must be performed before the multilayered wiring portion having the organic polymer layer as the insulating layer is formed. For this reason, processing such as etching in the manufacturing process of the multilayered wiring film having the organic polymer layer as the insulating layer may damage the leads or pins, and a special jig for protecting the leads or pins from an etching solution is required. In addition, since the pins and leads project from the substrate surface, in a process of forming the multilayered wiring film having the organic polymer layer as the insulating layer or in the step of mounting an integrated circuit, a conventional jig for mounting a substrate cannot be advantageously used. This drawback may increase production cost.
There is a method of connecting leads at a low temperature without adversely affecting other portions. In this case, however, when an MCM is to be externally mounted, special processing in which the MCM is mounted at a temperature lower than that at which the leads are connected must be performed. In this case, production cost is increased.
In addition, when a flexible wiring substrate is mounted on the lower surface of a silicon substrate in a silicon-on-silicon MCM, special processing for connecting the upper surface of the silicon substrate to the lower surface of the silicon substrate must be performed. For example, a method of forming small through holes in the silicon substrate and forming conductive pillars in the holes is known. In this method, a electrodes are necessarily arranged at positions of the upper surface of the silicon substrate corresponding to the flexible wiring substrate connection portion on the lower surface of the silicon substrate, which restricts the area of the chip mounting surface. In addition, production cost is increased.
In addition, when input/output operations are performed through pins and leads, a characteristic impedance cannot be easily controlled. This method cannot be easily applied to a high-speed signal.